1. Field of the Invention
The invention concerns an electrochemical sensor with an integrated structure for the measurement of concentrations of reactive species and, more particularly, to a sensor made with thin layer or thick layer type technologies on a substrate that is electrically insulating and chemically inert at high temperature.
The invention can also be applied, in particular, to the making of an oxygen concentration sensor which can be used, inter alia, in the automobile industry.
2. Description of the Prior Art
One of the well-known groups of electrochemical sensors works on the principle of the concentration cell and measures the partial pressure of one or more species of the gaseous mixture to be analyzed. This gaseous mixture, which is present in a first compartment and is, for example, an inert gas/oxygen mixture, is separated from a reference medium by the wall of a solid electrolyte, each face of which has an electrode. As is well known, the equations that govern the working of these sensors are: at the electrodes/electrolyte interfaces: ##STR1## the voltage V.sub.E1/E2 which then develops between the electrodes is given by Nernst's law: ##EQU1## with R=perfect gases constant=8.314 J.(mole.K).sup.-1
F=Faraday No.=96490 Coulombs PA0 T=absolute temperature in degrees Kelvin PA0 P.sub.1 and P.sub.2 =partial pressures of media 1 and 2 in the compartments 1 and 2. PA0 differential expansion between the alumina support heated by the exhaust gases and the corundum substrate, heated to a temperature which is generally different by the resistor incorporated in its rear side. This entails repeated shearing of the metallurgical link between the rear side of the sensor and the alumina strip, which may lead to a break in the mechanical link between the sensor and its strip. PA0 adsorption of water (always present in the exhaust gases) on the surface of the alumina strips for a temperature below 650.degree. C., the effect of which is to make these strips conductive on the surface and, therefore, to short-circuit the deposited metallic tracks. This problem can be partially resolved by passivating the surface of the alumina strips and the metallizations by means of an impervious refractory enamel, but the absorption of water will occur, nonetheless, at the place where the passivation ends, namely at the place where the thermocompressed connection wires coming from the sensor are connected to the corresponding pads screen printed on the alumina strips. PA0 miscellaneous, electrically conductive deposits due to the cracking of the additives used in the oil and fuels, additives which, despite the above-described passivation, also have the effect of short-circuiting the platinum connection at the place where they are thermocompressed on the corresponding pads screen printed on the alumina strips. PA0 an elongated substrate comprising, lengthwise, a first detection zone, a second zone for the supporting and fixing of the sensor and a third electrical connection zone; PA0 one or more electrochemical cells, at least one of which is sensitive to an excess level of one of the reactive species with respect to a defined stoichiometry, said cell being implanted directly in the first zone of he substrate; PA0 electrical connection areas located in the third electrical connection zone; PA0 conductive tracks of electrical connections deposited on the substrate and connecting the electrochemical cell or cells to the electrical connection areas located in the electrical connection zone; PA0 an encapsulating layer of material impervious to said gaseous mixture covering at least the first and second zones, said layer encapsulating, in particular, the electrochemical cells and the connection tracks and having at least one aperture for the entry of gases towards the electrochemical cell; PA0 a fixing part placed in the second fixing zone and preventing fluid from flowing from the first zone to the third zone.
Thus, knowledge of the temperature and of one of the partial pressures enables the unambiguous determining of the other partial pressure.
Should the mixture be reactive, for example, if it is a mixture of O.sub.2 +CO, and if the electrode is a catalyst of the reaction of these gases, the following reaction occurs: EQU 2CO+O.sub.2 .revreaction.2CO.sub.2 ( 3)
and, finally, if the combustion is complete until reversible thermodynamic equilibrium is achieved, the following relationship is verified: ##EQU2## with K(T) being a coefficient of equilibrium dependent on the temperature, and P CO, P O.sub.2, P CO.sub.2, being the partial pressures of carbon dioxide, oxygen and carbonic gas.
In applications concerning the regulation of automobile engines with spark ignition, in order to determine the partial pressure of oxygen at the exhaust (medium 1 for example), knowing the reference pressure (medium 2 which is generally air) in removing the need to measure or regulate the temperature, use is made of the fact that, if the exhaust gases are brought to thermodynamic equilibrium (end of combustion), the value of the partial pressure of oxygen, as shown in FIG. 1, varies by about 15 orders of magnitude when the mixture feeding the cylinders passes through the stoichiometric state.
Thus, in the above-described Nernst formula, a voltage leap is observed when the mixture passes the stoichiometric state .DELTA.V=(RT/4F) log PO.sub.2.sup.rich /PO.sub.2.sup.poor); if the temperature is in the range of 800.degree. C., the term RT/4F is of the order of 50 mV and the .DELTA.V will be greater than 750 mV.
Sensors of this type, called stoichiometrical sensors, generally consist of a glove finger made of stabilized zircon. The external wall, provided with a porous platinum electrode (measuring electrode), is in contact with the gas for which it is sought to analyze the oxygen content and the inner wall, also provided with a platinum electrode (reference electrode), is in contact with a reference gas, generally air. The platinum of the measuring electrode catalyzes the end of combustion of the exhaust gases for example and, in order not to saturate the platinum, it is encapsulated by means of a porous diffusion layer, the main effect of which is to limit the flow of gases reaching the catalytic sites of the platinum electrode.
FIG. 2 shows a few typical responses of these glove finger sensors using air as a reference.
However, the making of sensors of this type may take different forms. FIGS. 3 and 4 show examples of embodiments obtained from successive deposits (thin layers or thick layers) of ceramic and metallic materials on an electrically insulating substrate. According to FIG. 3, there is a known method to make an electrochemical sensor comprising a solid electrolyte EL on a substrate Sb. This electrolyte may be made of zirconium oxide, thoria or cerium oxide stabilized by one or more elements belonging to columns II.sub.A and III.sub.B of the periodic classification of elements. It may be made as a thin layer or a thick layer, or it may be massive.
Electrodes E1/P1 and E2/P2 are deposited on the electrolyte EL and on the substrate Sb. The electrodes E1/P1 and E2/P2 are located in one and the same plane. The electrode E1/P1 combines the functions of an electrode and a reference medium. The electrode E1/P1 is further protected from the external environment by an impervious and inert insulating material S1 which coats it. It is possible, for example, to use an association of the type Ni/NiO or Pd/PdO to make this electrode/reference medium. The electrode E2/P2 has two zones and communicates directly with the medium to be analyzed in which there flows the gaseous mixture G through a hole made in the insulating body S1 which also covers it. In the first zone Ct, the electrode is not in contact with the electrolyte EL. The fluid to be analyzed must flow through the zone Ct which takes the place of a catalyst and a test sample inlet chamber. In this zone, the reactive species of the mixture to be analyzed (for example, in the case of exhaust gases: CO and O.sub.2) are brought to complete thermodynamic equilibrium before they have reached the electrochemical cell itself: EQU E2/P2-EL-E1/P1
P2 represents the partial pressure of oxygen after catalysis in the real medium to be analyzed. The catalysis, which enables obtaining thermodynamic equilibrium, is achieved by the fact that the fluid flows through the catalyst in a direction parallel to the plane of the electrodes. The electrodes are extended outwards by metallic links to which the contacts C1 and C2 may be soldered. These links are made with platinum veneer for example. In one practical embodiment, the metallic links and the electrodes are made so as to form a single part. The substrate Sb may consist of a material (such as corundum) which insulates well at the operating temperature of the device and gives the unit mechanical strength. The face of the substrate 1 opposite the electrochemical cell has a heating resistor RC which enables accelerating the reaction.
The deposits can be made by well known techniques, such as: vacuum deposition (cathode spraying, evaporation), vapor phase deposition, electrochemical deposition or ion implantation or by a combination of two or more of these techniques. For a metal/oxidated metal reference mixture, such as Pd/PdO, the response, in voltage, to a temperature of about 800.degree. C. is shown in FIG. 2 for the corresponding temperature (at 800.degree. C., the pressure of equilibrium of the Pd/Po mixture is equal to 0.2 Atm.).
The descriptions of sensors thus made will be found in the French patents Nos. 2441 164 and 2 444 272.
FIG. 5 shows another embodiment of a sensor according to the prior art.
This figure repeats the elements illustrated with reference to FIG. 3: the measuring cell E1/P1-El1-E2/P2, deposited in thin or thick layers or in massive form on a substrate Sb, the catalysis region Ct and the test samples inlet region P.sub.es where the interactions with the gaseous mixture to be analyzed take place. In fact, in the example described, these latter two regions consist of an extension of the measuring electrode E2/P2. The output signal VS of the sensor is transmitted to external circuits (not shown) by connections C1 and C2. The two electrodes E1/P1 and E2/P2 should at least be shielded by an impervious and inert insulating jacket S1, made of enamel for example.
According to the sensor of FIG. 5, an additional electrochemical cell is integrated into the sensor and comprises a solid electrolyte El2 inserted between two electrodes E3 and E4. In the exemplary embodiment of FIG. 5, and according to the first approach, the second electrode E4 is identified with the extension of the measuring electrode E2. The cell is flush with the surface of the insulating material S1 so as to communicate with a medium containing oxygen. This medium may be the medium Mex in which there flows the gaseous mixture G to be analyzed. The cell E3-El2-E4 is supplied with a control current Ip by means of the connections C3 and C4, C4 being identified wth C2. The substrate face opposite to the electrocemical cell also has a heating resistor RC.
Referring again to the above description, it is immediately seen that the cell E3-El2-E4, working as an ion pump, modifies the oxygen composition of the test sample let into into the sensor, namely the oxygen composition of the gaseous mixture flowing towards and through the catalysis zone Ct to subsequently reach the measuring cell E2/P2-El1-E1/P1, and does this modification as a function of the amplitude and bias of the current Ip. It follows therefrom that this cell produces an output signal VS which flips over, no longer when the stoichiometric state of the mixture G is reached but "before" or "after" said stoichiometric state, the lag on either side of the stoichiometric state being defined continuously by the amplitude and bias of the control current Ip. FIG. 6 shows some typical responses of this type of sensor as a function of the bias current Ip.
A description of a sensor of this type will be found in the French patent No. 2 494 445 and 2 442 444.
In the present state of the art, the sensors, which have rectangular dimensions of 8 mm.times.2 mm, are manufactured collectively on square (3 inch.times.3 inch) or circular (.0.=3 inches) corundum substrates which are then cut out with a diamond saw or by laser (CO.sub.2 or YAG). Each sensor is then mounted and wired to a flat support (strip) of alumina or any other electrical insulating material which enables the conveying of the various electrical input and output signals, through electrically conducting tracks, between the medium to be analyzed and the electronic control elements for the composition of the fluid mixture or the measurement of the oxygen rate. This strip supporting the sensor is itself then mounted in a cylindrical insulating element made of ceramic which enables it to be matched to the inside of a metallic part similar to an automobile spark plug body. The connections between the sensor and the external medium are made by means of electrical conducting tracks deposited on the supporting strip and connected to the other end of the strip to a connector. For example, when checking the air/gasoline mixture feeding the automobile engines, the medium analyzed consists of gas flowing in the exhaust silencer, and the data given by the sensor implanted in the exhaust gas flow is processed by an electronic system which then acts on the gasoline injection nozzles. The above-described assembly therefore enables the electrical signals to be conveyed between the sensor located inside the exhaust silencer and the various wires and electrical elements serving the central electronic system which controls the injection nozzles.
In greater detail, sensors are currently assembled with a platinum compound which is annealed at 900.degree. C. The platinum provides, after the heat treatment, firstly a metallurgical link between the rear side of the sensor and the alumina strip which enables the sensor to be mechanically supported and, secondly, an electrical link between the heating resistor of the sensor on the rear surface of this sensor and the heating current leads supported by the rear side of the alumina strip. The signal output wiring of the pump (in a poor mixture sensor as described with reference to FIG. 5) and the ground is done by means of platinum wires with a diameter of approximately 50 .mu.m, soldered by thermocompression firstly to the outputs of the sensor and, secondly, to the pads made for this purpose on the alumina strip. These two operations, assembly and wiring, are nevertheless lengthy and difficult and, in addition, entail a number of specific problems, namely:
The invention provides a sensor which can overcome these problems.